The present disclosure relates to an image-forming apparatus for scanning to expose a photosensitive drum to a laser to form an image (toner image).
Conventionally, a rotating polygon mirror may be made to reflect a laser beam and a lens, mirror, or the like may be used to form an electrostatic latent image on a photosensitive drum, thus forming a toner image. When such a laser beam is reflected and scanned to expose the photosensitive drum, it is necessary to appropriately control a variety of parameters, such as the start timing for the scanning and exposure of each of the lines and the rotational speed of the polygon mirror.
One known example is an image-forming apparatus for causing a rotating polygonal mirror to reflect an exposure beam controlled in accordance with an image clock and using a rotating process member to form/develop/transfer an electrostatic latent image and obtain an image, wherein the speed variance of the process member is detected and the rotational speed of the rotational polygonal mirror, the frequency of the image clock, and the light intensity of the exposure beam are controlled in accordance with the detected speed variance.
Some image-forming apparatuses form a toner image in each color on a plurality of photosensitive drums and superimpose the toner images in each color to carry out color printing (also sometimes called a tandem scheme). Because of the superimposition of the toner images in each color, the toner images in each color sometimes have uneven densities and/or misaligned positions, whereupon the image quality is degraded. In view whereof, the phases of the polygon mirrors (polygon motors) for each color are sometimes matched.
For example, the inputting of a print start command into an operation panel, the receipt of image data, or the like serves as a trigger for printing to start. In association with the start of printing, the polygon motors for each color, which rotate the polygon mirrors, are rotated up to a reference speed that has been determined in advance. Then, in order to be able to match the angle of rotation (phase) of the polygon mirror (polygon motor) for a color serving as a reference, the rotational speed of the polygon motors for the colors other than the reference color (colors targeted for phase correction) are intentionally shifted from the reference speed. The phase difference is continually reduced and, once the phase difference reaches an acceptable range, the polygon motors for the colors targeted for phase correction are returned to the reference speed.
From the standpoint of the ability of the polygon motors to track and respond to speed variance, it is desirable to lower as much as possible the amount of change from the reference speed to change the rotational speeds of the polygon motors of the colors targeted for phase correction, in order to be able to accurately and precisely match the phases of each of the polygon mirrors. For this reason, during the correction of the phase difference, it is more preferable for the rotational speed of the polygon motors not to be over-shifted with respect to the reference speed.
On the other hand, from the standpoint of starting printing as quickly as possible, it is desirable to rapidly complete the correction of the phase differences of each of the polygon motors and to shorten the time needed to bring an exposure device to a state where printing is possible. However, when the amount of change from the reference speed is lowered as much as possible and the rotational speed of the polygon motors for the colors targeted for phase correction is changed to correct the phase difference, a problem emerges in that it is a long time until the phase difference between the polygon mirror (polygon motor) of the reference color and the polygon mirrors (polygon motors) of the colors targeted for phase correction falls within the acceptable range.
With the above-mentioned known image-forming apparatus, a variety of processes and controls such as for changing the image clock and changing the intensity of the exposure beam are performed to prevent a degradation in the quality of the image. However, there is no adjustment for the phase difference between the polygon mirrors (polygon motors) for each color. As such, when the amount of change from the reference speed is lowered as much as possible and the rotational speed of the polygon motors for the colors targeted for phase correction is changed to correct the phase difference, it is not possible to address the problem which emerges in that it is a long time until the phase difference falls within the acceptable range.